Breach (Water Overlay)

A breach is a modification to the terrain height, with an optional in- or outflow of water for the hydrological model. This can be used to represent calamitous situations, such as a breach in a levee. Breaches can also be used to easily simulate a terrain height increase, effectively creating a levee.

A breach can either be defined solely as a terrain height modification using its BREACH_HEIGHT attribute, or as a connection to a water body outside of the hydrological model by adding attributes related to the external water body. If the breach is only defined as a terrain height change, only water that is already created or defined in some other way in the hydrological model can flow through and from it.

If the breach is given the attributes required for the external water body, water will automatically be created or removed uniformly on the breach, depending on the simulated water body "behind" the breach.

The breach can grow over time, based on its initial width and the critical speed at which water may flow. If a BREACH_WIDTH is defined, the breach's polygon is intersected with a circle emanating from the centerpoint of the polygon. It is only in that intersection, dubbed the "active breach", that water will flow in from the simulated external water body.. The radius of the circle defining the intersection will expand when the water flowing in from the external water body exceeds the BREACH_SPEED. Water flowing in the hydrological model across the breach's surface without explicitly entering or leaving the hydrological model through the breach and its simulated water body does not "count" for the critical speed. All cells in the active breach are considered directly adjacent to the external water body.

The entire breach area is considered lowered to the height defined by its BREACH_HEIGHT attribute, even if the active breach does not extend across the entirety of the breach area.

Water flowing through or across a breach will flow in the direction defined by the BREACH_ANGLE attribute, regardless of whether the water flowed onto the breach from elsewhere in the water model, or from the simulated external water body. If no BREACH_ANGLE is defined, water can flow in any direction.

If no critical speed is defined, the active breach will never grow. If no width is defined, the width is assumed to be very large, creating an intersection exactly the size of the polygon.

Attribute	Unit	Description	Default (when attribute is not present)
BREACH_HEIGHT	m + datum	The new terrain height at the location of the breach.	n/a
BREACH_WIDTH	m	The radius of the breach as drawn on the polygon defining the breach, emanating from the center point.	10000
BREACH_ANGLE	geo angle (0-360°)	The angle at which water can flow from the breach.	n/a
BREACH_SPEED	m/s	The speed at which water should flow through the breach before the width begins to increase.	n/a
EXTERNAL_SURFACE_LEVEL	m + datum	The height of the bottom of the external water body behind the breach. The lowest level the external water level can lower to.	0
EXTERNAL_WATER_LEVEL	m + datum	The water level of the external water body behind the breach. The initial level the water level is set to.	0
EXTERNAL_AREA	m²	The surface area of the external water body behind the breach. The larger the external water body, the more water can flow from it, and the smaller the effect of the inflow or outflow of water on the external water level.	n/a